PROJECT SUMMARY Knee osteoarthritis (OA) is a highly prevalent, leading cause of pain that limits physical functioning in older adults. Although clinical practice guidelines recommend physical exercise for managing symptoms of knee OA, several systematic reviews indicate that exercise intervention studies provide only short-term benefits (lasting <6 months) for knee OA. Many older adults with painful knee OA return to sedentary lifestyles when structured exercise programs end. Indeed, studies show that physical activity levels are low in this large, rapidly growing population. Activity restriction (e.g., ?resting?) is a key challenge to address as it is a common strategy older adults use to reduce or prevent pain. Further, pain in older adults is strongly associated with fear of falling and other comorbid symptoms, such as depressed mood, fatigue, and difficulty sleeping, all of which further limit activity. Thus, efforts to increase and sustain physical activity in the geriatric knee OA population will require restructuring maladaptive beliefs about exercise and pain, as well as promoting self-regulatory skills to develop confidence to maintain an active lifestyle and manage symptoms that often limit activity. Therefore, we developed a group-based behavioral health (BH) program for older adults with painful knee OA that complements Enhance Fitness (EF) ? a multicomponent, community-based exercise program for older adults, involving balance, strength, and endurance training. The BH program (1) teaches coping skills to address pain and related symptoms, emphasizing self-management and self-efficacy; (2) addresses fear of falling by restructuring misconceptions to promote a view that falls risk is controllable; and (3) promotes walking behavior by (a) using Fitbits for objective self-monitoring and goal-setting, (b) helping discouraged participants reframe attributions of failure from uncontrollable to controllable causes, and (c) using group-based problem-solving of barriers to physical activity. Combining exercise with BH skills training can have reinforcing effects by improving functional capacity, reducing pain, and increasing self-efficacy for managing arthritis symptoms and achieving personal activity goals. Accordingly, our objective is to conduct a single-blind, randomized controlled trial in 280 community-dwelling adults age ?65 years with painful knee OA to determine the efficacy of a 4- month intervention of EF+BH versus an active control group of EF and health education (EF+HE). We hypothesize the EF+BH intervention will produce greater improvements in accelerometer-measured minutes of moderate-to-vigorous physical activity (primary outcome) than compared with the EF+HE control group at 6- month follow-up. Secondary outcomes include knee pain severity, knee pain-related functional limitations, mobility function, and fatigue. We will also examine the mediational role of short-term changes (over 4 months) in measures of self-efficacy and pain coping with physical activity outcomes at 6-month follow-up. If successful, the EF+BH intervention has high potential for dissemination as a community-based, non-pharmacologic pain management program for older adults with knee OA.